1. Field of the Invention
The present invention relates to an image sensing system and a correction method.
2. Description of the Related Art
Image sensors typified by a CCD image sensor and a CMOS image sensor have higher sensitivities to infrared light than the spectral sensitivity characteristic (spectral luminous efficiency) of human eyes. Hence, an infrared cut filter can generally be arranged between a camera lens and the image sensing surface of an image sensor. A dichroic filter, for example, can be used as the infrared cut filter.
A principal ray which reaches the center of the dichroic filter upon passing through the camera lens is perpendicular to the dichroic filter surface, whereas that which reaches the periphery in the plane of the image sensing surface of the image sensor makes a certain angle with the dichroic filter surface. The larger the distance from the center of the image sensing surface (i.e., the higher the image height), and the smaller the exit pupil distance of the camera lens, the larger the incident angle on the dichroic filter surface becomes. In this manner, a dichroic filter has a characteristic with incident angle dependence, so the intensity of light which strikes the image sensor upon passing through the dichroic filter differs depending on the image height. This generates color shading (color variation) in the sensed image.
To correct the color shading, an apparatus described in Japanese Patent Laid-Open No. 2008-153848 detects a fluorescent flicker to distinguish between a fluorescent light source and other light sources, and adjusts a color shading correction coefficient in accordance with the type of light source.
However, in the apparatus described in Japanese Patent Laid-Open No. 2008-153848, the type of light source is determined using information from the image sensor, so no information regarding a band outside the wavelength range defined by the cutoff wavelength of the infrared cut filter is taken into consideration, and this determination is influenced by the incident angle dependence of this filter. Therefore, the luminance of an object in a wavelength range in the vicinity of the cutoff wavelength relative to the average luminance of the object cannot be detected with high accuracy, and this may lead to excessive or poor correction.